1. Field of the Invention
This invention relates to an improved process for preparing dinitrotoluene by reacting nitric acid with toluene. More particularly, the invention relates to a liquid-phase reaction of toluene with a large excess of concentrated nitric acid at selected conditions to minimize explosion hazards and produce a product which has a substantially reduced by-product content. Dinitrotoluene synthesis with low by-product content avoids operations for product purification and the effluent water treatment problems associated with those operations.
2. Background Of The Invention
Commercial processes for preparing dinitrotoluene (DNT) react toluene with a mixed acid containing nitric and sulfuric acid. Mononitrotoluene (MNT) is produced first, followed by DNT formation. A variety of process schemes are used to increase conventional process efficiency. These schemes synthesize the product in a heterogeneous system comprising an organic liquid phase and an acid liquid phase. Nitration occurs in both phases and is Predominate at the phase interface. The reaction rate is controlled by mass transfer in the system.
During MNT synthesis, a small amount of toluene is oxidized instead of nitrated. An article in Albright & Hanson, Industrial and Laboratory Nitrations, ACS Symposium Series 22, Chapter 8, Hanson et al., Side Reactions During Nitration, pages 133 to 155, describes the oxidation by-products to be mostly cresol and phenol type compounds. Some of the compounds, such as dinitro-ortho-cresol, are highly toxic. These by-products are removed from the DNT by washing with alkaline water, which must then be treated to remove the toxic compounds before it is discharged into public waters. The problems caused by the by-products are further described by C. Hanson et al., supra, on page 133, as follows:
"By-Product formation represents loss of reactants or nitro product. It is also likely to result in increased costs for the separation and purification of the main product, such as increased capital costs for distillation and washing stages, together with higher operating costs for steam and treatment chemicals."
Additionally, the use of mixed acid systems usually involve reconcentration of the spent sulfuric acid after the nitration reaction. This reconcentration step is time consuming, energy intensive and requires the use of expensive materials of construction.
In view of these disadvantages associated with mixed nitric/sulfuric acid systems, there have been several developments in the prior art to perform gas phase or liquid phase nitrations with concentrated nitric acid in the absence of sulfuric acid. U.S. Pat. No. 2,362,743 discloses a two-step process for the manufacture of dinitrotoluene in the absence of sulfuric acid which comprises (a) nitrating toluene to mononitrotoluene using a nitric acid having a concentration from about 60% to about 75% and a mole ratio of toluene to nitric acid of about 1 to about 3.5 and (b) further nitrating the mononitrotoluene to dinitrotoluene using nitric acid having a concentration of from about 90% to about 100%, and a mole ratio of mononitrotoluene to nitric acid of about 1 to about 3. Although the process of this patent is advantageously conducted in the absence of sulfuric acid, it was found that in step (b), a very high percentage of the nitrated product (up to 25%) based upon the amount of toluene reactant employed does not phase separate from the nitric acid medium. The patent teaches vacuum distillation of the product mixture to isolate the desired dinitrotoluene, which is an expensive and highly energy intensive process step.
U.S. Pat. No. 3,928,395 describes a process for nitrating unsubstituted or substituted benzene at a reaction temperature of -40.degree. C. to 80.degree. C. using 90% to 100% nitric acid in the optional and preferred presence of a dipolar aprotic solvent, wherein the reaction is halted by means of a dipolar aprotic solvent.
U.S. Pat. No. 3,957,889 describes an improved process for nitrating toluene or ortho-xylene with nitric acid, the improvement being enhancing the rate of the nitration reaction by carrying it out in the presence of at least an effective amount of anhydrous calcium sulfate or soluble anhydrite.
U.S Pat. No. 4,064,147 describes the preparation of aromatic mononitro compounds (such as mononitrobenzene) by a liquid phase reaction with nitric acid having an acid concentration of between 70 and 100% by weight using a reaction temperature of between 0.degree. C. and 80.degree. C. When employing a relatively reactive compound such as benzene or toluene as a starting material, this patent teaches that a nitric acid concentration of between 70 and 90% by weight is preferred. The process of the patent requires a ratio of nitric acid plus water to organic components of not below 3 when using 70% nitric acid, and not below 8 when using 100% nitric acid. However, it has been found that such a high acid ratio using 100% nitric acid tends to favor dinitro-compound production, not desired by the process of the patent.
U.S. Pat. No. 4,804,792 describes the nitration of benzene and toluene by contacting these with concentrated nitric acid in the presence of a molten nitrate salt. The patent states that the molten salt serves as a temperature regulator for the reaction and as an isothermal medium for the reactants. A preferred method of contacting the reactants in the presence of the molten salt is stated to be by bubbling the reactants into a bath of the molten salt by means of a carrier gas such as nitrogen. The vapor phase reaction is stated to be carried out at a temperature of between 150.degree. and 250.degree. C.
U.S. Pat. No. 4,918,250 describes a process for nitrating toluene to DNT and phase separation of the product using an inorganic salt as a phase separation agent. In this patent, DNT is produced in a two-step liquid phase nitration reaction between nitric acid and toluene in the absence of sulfuric acid and solvent. In the process of the patent, an inorganic salt is incorporated into the mixture of DNT and unreacted nitric acid in an amount sufficient to cause phase separation of the mixture in order to facilitate isolation of the DNT from the unreacted nitric acid in the product mixture.
More recent developments and improvements in this field are disclosed in U.S. Pat. Nos. 5,001,272, 5,099,078, and 5,099,080. All these patents relate to the production of DNT by nitration of toluene with concentrated nitric acid using a molar excess of nitric acid. Thus the '272 and '078 patents call for a molar ratio, nitric acid to toluene, of up to about 9:1; whereas, a ratio ranging broadly from about 7:1 to about 20:1 is disclosed in the '080 patent.